42
Abstract —Unobserved mortalities of nontarget species are among the most troubling and difficult issues associated with fishing, especially when those species are targeted by other fisheries. Of such concern are mortalities of crab species of the Bering Sea, which are exposed to bottom trawling from groundfish fisheries. Uncertainty in the management of these fisheries has been exacerbated by unknown mortality rates for crabs struck by trawls. In this study, the mortality rates for 3 species of commercially important crabs—red king crab, (Paralithodes camtschaticus), snow crab (Chionoecetes opilio) and southern Tanner crab (C. bairdi)—that encounter different components of bottom trawls were estimated through capture of crabs behind the bottom trawl and by evaluation of immediate and delayed mortalities. We used a reflex action mortality predictor to predict delayed mortalities. Estimated mortality rates varied by species and by the part of the trawl gear encountered. Red king crab were more vulnerable than snow or southern Tanner crabs. Crabs were more likely to die after encountering the footrope than the sweeps of the trawl, and higher death rates were noted for the side sections of the footrope than for the center footrope section. Mortality rates were ≤16%, except for red king crab that passed under the trawl wings (32%). Herding devices (sweeps) can expand greatly the area of seafloor from which flatfishes are captured, and they subject crabs in that additional area to lower (4–9%) mortality rates. Raising sweep cables off of the seafloor reduced red king crab mortality rates from 10% to 4%.
Manuscript submitted 7 March 2012. Manuscript accepted 9 November 2012. Fish. Bull. 111:42–53 (2013). doi:10.7755/FB.111.1.4 The views and opinions expressed or implied in this article are those of the author (or authors) and do not necessarily reflect the position of the National Marine Fisheries Service, NOAA.
Quantification and reduction of unobserved mortality rates for snow, southern Tanner, and red king crabs (Chionoecetes opilio, C. bairdi, and Paralithodes camtschaticus) after encounters with trawls on the seafloor Craig S. Rose (contact author)1 Carwyn F. Hammond1 Allan W. Stoner2 J. Eric Munk3 John R. Gauvin4 Email address for contact author:
[email protected] 1
Conservation Engineering Program Alaska Fisheries Science Center National Marine Fisheries Service, NOAA 7600 Sand Point Way NE Seattle, Washington 98115
3
Shellfish Assessment Program Alaska Fisheries Science Center National Marine Fisheries Service, NOAA 301 Research Court Kodiak, Alaska 99615
2
Fisheries Behavioral Ecology Program Alaska Fisheries Science Center National Marine Fisheries Service, NOAA 2030 Marine Science Drive Newport, Oregon 97365
4
Alaska Seafood Cooperative 4241 21st Avenue W, Suite 302 Seattle, Washington 98199
The potential for unobserved mortality of crabs that encounter bottom trawls but are not captured has long been a concern for the management of groundfish fisheries in the Bering Sea (Witherell and Pautzke, 1997; Witherell and Woodby, 2005). Fisheries on the crab and groundfish stocks of the wide continental shelf of the eastern Bering Sea have made Dutch Harbor, the principal port for that area, the highest port by tonnage in the United States and 1 of the 2 highest ports by dollar value for more than 20 years.1 Three major crab species—red king crab (Paralithodes camtschaticus), snow crab (Chionoecetes opilio), and southern Tanner crab (C. bairdi)—are targets of large commercial fisheries (Otto, 1990). The 2 Chionoecetes species 1
U.S. Department of Commerce. 1995– 2011. Fisheries of the United States 1995 (1996,…,2011). Current Fishery Statistics 1995 (1996,…,2011). U.S. Dep. Commer., NOAA, Natl. Mar. Fish. Serv., Fisheries Statistics Division, Silver Spring, MD. [Available from http:// w w w. s t . n m f s. n o a a . g o v / c o m m e r c i a l fisheries/fus/index.]
have similar low, flat body shapes and inhabit deeper water with muddier substrates than that of the red king crab, which has a thicker body and inhabits shallower, sandier areas (Jadamec et al., 1999; Donaldson and Byersdorfer, 2005). Groundfish species, particularly gadids and flatfishes are targeted with trawls. Overlaps between crab habitat and areas trawled by groundfish fisheries can result in some mortality for crabs that encounter groundfish trawls, either through capture and discard (bycatch) or as unobserved mortality of crabs that remain on the seafloor (Witherell and Pautzke, 1997). The current management measures to control and reduce bycatch of the major Bering Sea crab species in Alaska groundfi sh fi sheries include extensive year-round trawl closure areas (Fig. 1) and bycatch limits outside these areas. The yearround closure areas were established to protect areas of known concentrations of female and juvenile crabs. Armstrong et al. (1993) and Witherell and Pautzke (1997) cited unobserved trawl–induced mortality, along with
Rose et al.:
Mortality rates for Chionoecetes opilio, C. bairdi, and Paralithodes camtschaticus after trawls on the seafloor
43
Pribilof Islands habitat conservation zone
Red king crab savings area
Nearshore Bristol Bay closure area
Figure 1 Sampling locations for snow (Chionoecetes opilio) (S) and southern Tanner (C. bairdi) (T) crabs in 2008 and red king crab (Paralithodes camtschaticus) in 2009 (RK), during our study of unobserved mortality rates from bottom trawling. Bottom trawl area closures (shaded) and depth contours are included for reference.
possible habitat degradation, as principal reasons for the establishment of these closures. Crab bycatch limits (on the basis of numbers caught) have also triggered additional closures if seasonal, species-specific (and sometimes area-specific) limits are reached. These bycatch numbers are obtained from onboard fishery observers on an in-season basis (Witherell et al., 2000). The species-specific crab bycatch limits (in estimated numbers of crabs brought aboard) are thought to have a biologically insignificant effect on the different crab populations because these limits have represented as little as 0.113% of the abundance index for snow crab and 0.5–1.0% of abundance for southern Tanner crab and red king crab (Witherell and Pautzke, 1997). Critics of the existing framework of measures for crab bycatch management have from time to time asserted that, although bycatch limits appear to be sufficiently conservative, bycatch represents only a fraction of the actual mortality of different crab species caused by groundfish fisheries. Citing an unpublished technical paper, Thompson (1990) estimated actual trawl gear mortality for king crabs to be “10 to 15 times the number of crabs that are caught in the net (and esti-
mated by [National Marine Fisheries Service] observers).” These concerns cannot be adequately evaluated without addition of valid estimates of the unobserved mortality rates for these crab species to the assessments of bycatch and discard. Some crab researchers in Alaska (Murphy et al., 1994) also have underscored the need for additional research on injury rates and unobserved or unaccounted for mortality from both directed crab fisheries and groundfish trawl fisheries. Dew and McConnaughey (2005) concluded that excessively high mortality rates on male Bristol Bay red king crab from the directed fishery and unaccounted for mortality of females from the groundfish fisheries explain the downward population trajectory of this crab species through the late 1970s and early 1980s better than does the more accepted scientific hypothesis that the low population levels of red king crab were explained by unfavorable climate conditions. Worldwide, the recognition of unobserved mortalities as a potentially significant element by the fishing industry and by fishery managers has increased the number of studies that have addressed such mortalities and the range of methods used in their estimation.
44
Broadhurst et al. (2006) provided a thorough review of such studies. Although a great number of studies estimated mortalities of discarded catch, others dealt with mortalities of escaping animals not brought aboard the fi shing vessel. Broadhurst et al. (2006) noted that studies of escaping animals, almost exclusively fishes, lately have emphasized methods where escaping animals are recaptured in cages that are then detached from the fishing gear while still at fishing depths. Those cages then are moved slowly to shallower depths, where they are maintained by divers long enough to assess delayed mortalities. Earlier methods involved capture of escaping animals in auxiliary nets before they were brought aboard and held long enough to evaluate mortality rates. However, stress and injury from recapture and extended towing and holding times could have easily masked or exacerbated the effects of the escape process, particularly for animals vulnerable to skin abrasion damage. More recent methods retain the experimental subjects in an environment closer to what they would experience after actual escape. The cost of these gains is that each collection of affected animals requires an extended series of activities that are time consuming and labor and resource intensive. These time and resource demands greatly restrict the number of experimental samples that can be collected and held and, hence, the number of experimental factors that can be addressed. As an experimental subject, crab are significantly different from fish for which the in situ capture, transfer, and holding methods were developed. Exoskeletons protect crabs from the type of abrasion to which fish are particularly susceptible during net capture and crowded holding. As a trawl net approaches, fish continue swimming, often to exhaustion, to avoid contact with the net and other animals, but crab, being much slower, can flee only briefly before being overrun (Rose, 1995). Another difference is how crabs interact with fishing gear. Broadhurst et al. (2006), describing research on fishes, noted, “Because most experiments have quantified escape mortality at the codend, the potential for mortalities as a result of collisions and escape through other parts of the gear have largely been ignored.” Because of the sizes and behavior of Bering Sea crabs and the configurations of Bering Sea bottom trawls, most crabs escape under the forward parts of trawl systems, and interactions typically last only a few seconds as the crab passes the components of the net that directly contact the seafloor. Rose (1999) studied crab mortalities after such escapes under the forward sections of bottom trawls through assessment of visible injuries to red king crab that resulted from passes of crabs under different trawl footrope designs. The crabs were recaptured in an auxiliary net fished behind the main footropes. A control footrope, suspended with floats to allow crabs to pass beneath with minimal damage, also was used. A low rate of injuries for control crabs indicated that recapture of crabs to bring them aboard could be
Fishery Bulletin 111(1)
done without greatly increasing injury to crabs. The principal limitations of that study were the following: 1) crabs were not held beyond the initial assessment of injuries to observe delayed mortality; and 2) observations were limited to crabs that passed under the center section of the footrope, a small portion of the area swept during trawling. Studying mortality of crabs discarded from trawl catches, Stevens (1990) effectively applied a strategy in which all subject crabs were assessed for selected condition attributes and a sample was held long enough to relate those attributes to delayed mortality. Since that study, such methods have been expanded and improved. Davis and Ottmar (2006) used assessment of a range of reflexes of Pacific Halibut (Hippoglossus stenolepis) to build a predictor of delayed mortality, the Reflex Action Mortality Predictor (RAMP). In a pilot study for this project, Stoner et al. (2008) found the RAMP technique effective for estimation of delayed mortalities for snow and southern Tanner crabs. Our research addressed unobserved mortality rates for 3 principal commercial crab species of the Bering Sea: red king crab, southern Tanner crab, and snow crab. We improved methods for collection of crabs immediately after trawl encounters as used by Rose (1999) and applied the RAMP technique as described by Stoner et al. (2008) to assess the mortality probabilities for crabs that passed under the sweeps, wings, and central footrope of a commercial groundfish trawl. Raised sweeps, which reduce seafloor contact yet maintain herding of flatfishes (Rose et al., 2010), also were used at the red king crab sites to evaluate whether they would reduce crab mortality rates. Observations of control animals collected with identical recapture nets but no trawl encounter were used to adjust observed mortality rates for effects of capture and handling.
Materials and methods A pilot study conducted in 2007 evaluated the RAMP and developed and tested techniques for 1) recapturing crabs after encounters with trawl components, 2) handling and assessing those crabs on deck, 3) holding selected crabs to determine their survival over several days, and 4) using the RAMP to estimate the mortality probability of each crab (Stoner et al., 2008). Our study followed those methods closely, and the following description summarizes them and highlights all modifications made to the methods of the pilot study for our later study. Experimental tows for southern Tanner and snow crabs were made in August of 2008 ~111 km (~60 nmi) east of Saint Paul Island (Fig. 1). All tows included a mix of both species. Red king crab tows were made in August of 2009 at 2 sites in Bristol Bay, about 22 km (12 nmi) west of Amak Island and ~65 km (~35 nmi) northwest of Port Moller. Operations were conducted aboard the FV Pacific Explorer, a 47-m, 1800-hp com-
Rose et al.:
Mortality rates for Chionoecetes opilio, C. bairdi, and Paralithodes camtschaticus after trawls on the seafloor
mercial trawler equipped with a trawl configured similarly to the one used by many of the bottom trawlers that are used in Bering Sea groundfish fisheries. The 2-seam trawl net had a 36.0-m headrope and a 54.6m footrope, which was made of 19-mm-long link +steel chain and equipped with bobbins 46 cm in diameter. The ~70-cm sections between bobbins were covered with 2 steel-chain toggles, weighing 6.4 kg each, rubber disks of 4–20 cm, and one 5-kg circular weight. Wing extensions, installed ahead of the forward ends of the footrope, were made of 20-cm disks strung over 19-mmlong link chain. The cables (sweeps) that ran forward from the trawl to the doors were made of 48-mm combi-
Figure 2 Diagram of the trawl net (not to scale) used in our study of unobserved mortality rates for snow crab (Chionoecetes opilio), southern Tanner crab (C. bairdi), and red king crab (Paralithodes camtschaticus), showing positions of recapture nets designed to retain crabs after contact with various trawl components. No more than 2 of these nets were fished during the same tow, and the control net always was fished separately. Illustration by Karna McKinney.
45
nation rope, a product made of both steel and synthetic materials and used by most Bering Sea flatfish trawlers. The red king crab study included tests of sweeps equipped with disk clusters spaced at 14-m intervals and raised the combination rope 7.5 cm above the seafloor. Rose et al. (2010) found that such raised sweeps reduced seafloor contact while still herding groundfish effectively. Crabs were captured immediately after contact with the components of the main trawl by small recapture nets fished behind these 3 gear regions: 1) at center of the footrope, 2) at the footrope wings (including their extensions), and 3) behind the sweeps (Fig. 2). These recapture nets were small trawls designed to minimize fish capture and maximize crab capture. The recapture nets used behind the wings and sweeps had unequal bridle lengths, which were adjusted until water passed perpendicular to the center of the headrope of each net, as observed with an underwater camera. An identical recapture net was fished ahead of the trawl as a control to assess damage and mortality due to handling. A rope between the sweeps ahead of the control net was necessary to avoid overspreading. That rope was raised 23 cm off the bottom of the seafloor to avoid affecting crabs. Only 1 recapture net was used at a time during every tow in 2008 to ensure that nets did not tangle when launched. Experience allowed us to expand to 2 nets (1 sweep and 1 footrope) at a time during some tows in 2009; however the control net was always fished alone because it would potentially have damaged crabs before they reached the footrope. The time required to change positions of the recapture nets on the trawls precluded alternating them between trawl components on a tow-by-tow basis; therefore, all tows that addressed each gear component were done in 1 or 2 blocks of sequential tows. To maximize holding times for crabs affected by the trawl, the control tows were done last. The codend of the main trawl was not closed because the tows were too short to represent typical mortality due to capture by the trawl, and catch volume was considered unlikely to significantly affect sweep and footrope mortality. Towing speeds were 3–3.5 kn. Tow lengths were kept short to minimize damage to crabs from the recapture process but varied from 7 to 25 min to capture sufficient numbers of crabs. These speeds reflect industry practice, and, although commercial tows last much longer, the shorter lengths of the tows in our study did not change the relatively brief interactions between individual crabs and the ground contact components of the trawl. The main trawl was monitored with trawl sonar, which would detect any significant net asymmetry, and video observations of ground-gear components were used to check for atypical contact with the seafloor. Tow sites (Fig. 1) were selected to provide adequate numbers of the targeted crab species during relatively short tows. Both snow and southern Tanner crabs were sufficiently abundant to be studied at a single site in 2008, but red king crab research in 2009 required an
46
additional site. Although one of the red king crab sites was in a closed area, both such sites were similar in depth and substrate to areas where Bering Sea groundfish fisheries encounter that species. If 95 mm; those large snow crab were nearly all males. Large snow crab were approximately twice as likely to die as smaller
50
Fishery Bulletin 111(1)
Footrope wing
Footrope center
Sweep
Figure 4 Estimates and 95% confidence intervals of rates of mortality for snow crab (Chionoecetes opilio), southern Tanner crab (C. bairdi), and red king crab (Paralithodes camtschaticus that resulted from contact with 1 of 3 different components of a bottom trawl representative of the gear used bottom trawl fisheries in the Bering Sea—the footrope wings or extensions, the center of the footrope, or the sweep—and, for red king crab only, a sweep raised off of the seafloor (Rose et al., 2010).
snow crab or as any size of southern Tanner crab, and this difference persisted across all gear components and control catches. Large red king crab had higher mortality than smaller king crabs (P
